Rotate objects around irregular shape
you can use the path constraint to do exactly this.


Select all the objects you want to follow the curve, and the curve last.
Press ctrl +   P  to bring up the parent menu. (see pic 1 )
Select path constraint. Follow path is the old way of doing this, but the path constraint is easy to change settings.
In the constraint panel, change the XYZ axis to match your objects rotation. (see pic 2)
You may need to clear the location and rotation of the blades ,depending on the location and any other parents the object has.


ALT  + G  (clear location)
ALT  + R  (clear rotation)
and/or ALT + O (clear origin) 

You can then use the offset on each item to change the position around the curve.
To change the speed, in the curve options you can change the frames (see pic 3), or the evaluation time in the curve editor.